Since I have had this car the bfakes have always been hard, running or not, car off I can pump the brakes forever with no change in feel. With it running they still seem hard, am I not getting an assist? They stop the car okay but not good enough for emergency use. Recently replaced all pads and shoes , freed stuck e brake and all is the same. When I first go her I had my head hanging out the passenger window listening for a noise and my wife tapped the brakes and almost sent me through the windshield but never worked like that again, any ideas where to start?
Slr has been removed
I have an 89 the self leveling had been removed before I purchased. Sill has hydraulic brake booster attached to master cylinder.
Does ABS work? The manual recommends testing it periodically by pressing and holding pedal firm on sand or gravel and feeling the juddering of the ABS unit as it pulsates the brakes,
Also on startup… after car rolls and hits 3 MPH the ABS unit should pulsate and you should feel it in the pedal…
As for brakes being intermittent… maybe some pistons on calipers are hanging up?
Jack up the rear spin the wheels and test the rear pistons softly gradually the wheels should stop about the same position on brake pedal if not maybe one side is semi frozen,'check frozen caliper pistons. And when pedal is released wheels should spin easy with motor at idle in drive… Fronts jack up and have someone apply brakes while you hand spin wheel… do pistons and pads grab and release nicely??
Hopefully and I assume all disks are CLEAN…
It is hard to imagine the hydraulic booster pressure would change much either it has enough pressure or it does not… for it to be intermittent does not add up considering it is an all mechanical system a radial 3 piston pressure pump and mechanical booster cylinder…
take car for short drive don’t hit the brakes are any of the 4 rotors hotter than the others? that would indicate a drag of a caliper piston… IR heat gun is a neat tool to test this out.
If you hit the brakes hard, you cant lock up (ABS will kick in and pulsate) any wheel or wheels?–manuals sand gravel ABS test would be good.
DO you have extra wide tires on car?? If so the measly JAG sized rotors will have a hard time locking up large extra wide tires.—just a thought—
Any Castrol mineral Oil leaks? — used for the pump and brake booster—
Hope it helps
Brad 89 XJ6
Is there a relay that controls the brake assist?
My system has no electric pump, it is engine driven and apperently runs all the time the engine is on.
Some times I get a crunching noise at the pedal and it seems to go further to the floor but mostly the pedal seems hard.
Possibly. On the same block with black accumulator sphere are two switches, one for low pressure warning, the other one called “charging switch”. Someone may need to peek into electrical schematic to see where that goes.
The switches themselves do fail, so you may want to test them, first, as it is the least amount of work.
If everything is still on the car, I’d de-pressurize the brake assist, engine off, pushing on the brake two dozen times (yours already behaves unpressurized 
), unplug the green switch connector and measure with continuity tester (ohmmeter). Yellow-red and one of the black wires is low pressure warning, the yellow-black and the other black is charging. If you find first one open and the other closed, I’s say that part is not a prime suspect, and move on.
If off car, on the bench, low pressure warning switch is open, and charging is closed. You can poke a pin into either and they each reverse state to closed and open respectively. It takes a VERY strong poke, since they are operating at the 100 bar range (around 2000 psi).